The present invention refers to processes of recovering BHT values from the mother liquors of the crystallization of BHT obtained by alkylating p-cresol with isobutylene and, more particularly, it is related to a novel process of recovering said BHT values from mother liquors which contain relatively high concentrations of crystallization inhibiting compounds such as diisobutylene and lower and higher polymers of cresol preventing further crystallization of BHT.
In the past is has been very well known that 2,6-di-tert-butyl-p-cresol, also known as butylated hydroxy toluene or BHT, can be easily obtained by alkylation of p-cresol with isobutylene in the presence of a suitable acid catalyst such as sulfuric acid or phosphoric acid. This process for producing BHT, however, comprises as the last step a crystallization operation in order to remove the 2,6-di-tert-butyl-p-cresol from the reacted mixture obtained by the alkylation reaction with normal yields of BHT of from 75 to 80% of the theoretically possible obtention of BHT from the starting materials.
The mother liquors of the BHT crystallization step generally contain high proportions of diisobutylene and lower and higher polymers of cresol, together with realkylatable material consisting of residual p-cresol and monobutylated hydroxy toluene (tert-butyl-p-cresol) whereby it has been thought that realkylation of these BHT values could be effected to form additional BHT and thereby increase the amount already contained in said mother liquors to be furtherly crystallized. However, it has been shown that, while realkylation of the monoalkylated hydroxy toluene and of the residual p-cresol contained in the mother liquors can be easily effected by the very well known catalytic alkylation reaction described above, it is practically impossible to crystallize the thus obtained BHT from the resulting realkylated mixture, in view of the fact that apparently the presence of large proportions of diisobutylene and of lower and higher polymers of cresol, act as crystallization inhibiting compounds which prevent said crystallization.
In view of the above, regardless of the fact that the BHT values extant in the mother liquors of crystallization of the BHT-process clearly indicate that realkylation of said mother liquors could be effected in order to recover said BHT values, the existence of said crystallization inhibiting compounds (diisobutylene and cresol polymers) has prevented hereinbefore the use of these mother liquors in order to recover the BHT values contained therein, thus forcing the manufacturers to consider these mother liquors as waste materials which should be disposed of at a relatively high cost.
In view of the above, a process for the removal of the diisobutylene and lower and higher polymers of cresol from mother liquors obtained from the crystallization of BHT from the reacted mixture of p-cresol and isobutylene has been for long sought, which could economically remove said objectionable materials and thus provide a mother liquor which could be realkylated to recover the BHT values contained therein in large proportions.
Serious attempts have been made in the past to try to remove these objectionable crystallization inhibiting materials from the mother liquors of the BHT-process, without any success, inasmuch as all said processes relied mainly on the fractional distillation of the materials, which is a rather expensive procedure and does not provide for the proper reduction of the lower and higher polymers of cresol, regardless of the fact that said distillation process indeed reduces the contents of diisobutylene in the mother liquors. In other words, a vacuum distillation process of the mother liquors generally considerably reduces the content of the objectionable diisobutylene, but has the undesirable side effect of furtherly polymerizing the existing cresol and the monobutylated hydroxy toluene, forming additional amounts of lower cresol polymers and also has the objectionable side effect of furtherly polymerizing the lower polymers of cresol to produce certain amounts of higher polymers of cresol which are still more objectionable as to the properties of the mixture to permit efficient crystallization of the BHT which may be produced by realkylation of the BHT values contained therein.
The vacuum distillation procedures used heretofore, therefore, have not been successful, and certain other attempts involving reactions with proper reactants to precipitate the polymers of cresol have also been unsuccessful, thereby rendering said processes unuseful in industry, whereby the said mother liquors of the BHT-process have remained as waste materials without possibility of any further use for the recovery of the BHT values contained therein.